There is no specific plugin for Royal Mail. I think they don't have an API to dynamically calculate the shipping rates like UPS, FedEx or USPS.
So you need to manually create shipping methods for the different shipping fees you want to support on your website.
Note that BrainForge has an improved manual shipping plugin available here to better support UK post codes if you need to have shipping fees based on UK post code ranges:
brainforge.co.uk/shop/product/5-plg_hikashopshipping_bfmanual

Well, yes and no.
The APIs available there allow you to create an integration to send the order details to Royal Mail and get back a label (so that you can print the label to put on the package) and track the package.
It's basically the same function that can be done with the MyParcel shipping plugin:
www.hikashop.com/marketplace/product/245...shipping-plugin.html
So it doesn't help to calculate the shipping fees automatically. You would still need to manually enter the rates like before.

Well, the MyParcel plugin is for the MyParcel shipping system.
So it would require developing a Royal Mail plugin for the Royal Mail shipping system.
It was just to give an understanding of what such plugin could do.
You could modify the MyParcel plugin for the Royal Mail plugin, yes. That would basically mean rewriting the whole plugin besides a few lines of code for the structure of the plugin. It's like payment plugins: they are all based on the same structure. So you can use the PayPal payment plugin to write the Stripe plugin, but if you look at their code, besides a few lines of code which the same because they use the same HikaShop triggers, same HikaShop variables/functions, most of the code is actually different.

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